In a vehicle, there are provided: an automatic transmission including a shift gear mechanism that transmits drive force at a predetermined gear ratio from an input shaft side to an output shaft side by selectively engaging a plurality of frictional engagement elements (clutches or brakes) with one another; and an oil pressure control device that controls an oil pressure for engaging the above-described frictional engagement elements with one another. Then, in order to realize a plurality of shift speeds by combinations of engagement/release of the respective frictional engagement elements, the oil pressure control device of the automatic transmission, which is as described above, includes: a regulator valve (line pressure adjusting means) that adjusts a line pressure serving as a basic pressure, which is for engaging the frictional engagement elements with one another, from an oil pressure of hydraulic oil supplied from an oil pressure supply source; and a linear solenoid valve that adjusts an oil pressure (required oil pressure) for engaging any of the plurality of fictional engagement elements by engagement force, which is required thereby, by adjusting the oil pressure of the hydraulic oil adjusted to the line pressure by the regulator valve.
Then, in the oil pressure control device as described above, for example, as shown in Patent Literatures 1 and 2, there are ones configured to be capable of switching the line pressure, which is adjusted by the regulator valve, in order to enhance fuel efficiency of the vehicle. That is to say, each of oil pressure control devices described in Patent Literatures 1 and 2 includes a linear solenoid valve for switching the line pressure, and is configured to switch the line pressure, which is adjusted by the regulator valve, in two stages of a high line pressure and a low line pressure by on/off control of the linear solenoid valve concerned.